In the field of magnetic resonance (MR) imaging, it is known to especially use two types of radio frequency (RF) coils or antenna devices for exciting nuclear spins within a subject of interest, usually a patient, and detecting signals from them. Birdcage coils and transverse electromagnetic (TEM) coils are widely used for MR imaging in the very-high RF band (VHF) and have been introduced in commercial 3 T MR imaging scanners. The birdcage coil has multiple conductor rungs which are arranged around an examination region of the MR imaging scanner, extending parallel to a main magnetic field direction and connected to annular antenna rings. The TEM coil comprises multiple TEM coil elements, each having an elongate strip sections, wherein the plurality of TEM coil elements are arranged such that the elongate strip sections are substantially parallel and spaced apart in and/or around a space or volume for transmitting to or receiving from a subject to be examined. The strip sections are at their longitudinal ends electrically coupled to a conductive shielding of the RF antenna device. These coils require individual RF power supply to provide acceptable clinical image quality due to individual dielectric loading by the subject of interest.
The RF antenna device is usually provided with a fixed 90° excitation at the same amplitude which is provided by a hybrid coupler. Such coils have two feeding ports and are also referred to as quadrature coils, since they enable the generation of two orthogonal magnetic fields. Quadrature coils provide an increased signal to noise ratio of the RF antenna device. Other RF antenna devices even offer complete freedom of amplitude and phase for RF-shimming with a two-channel coil, which uses two geometrically decoupled feeding positions. This technique improves field homogeneity and enables imaging for different applications at high magnetic fields.
U.S. Pat. No. 7,345,481 B2 discloses an RF coil for an MR imaging system. The RF coil includes a birdcage section having a plurality of parallel spaced apart conductors and one or more cross or end conductors aligned generally transverse to the spaced apart conductors, and a TEM section having a plurality of parallel spaced apart conductors and a radio frequency screen. The birdcage section and the TEM section are relatively disposed with the parallel spaced apart conductors of each section aligned and define a subject receiving region.
However, there are still some subjects of interest in which the homogeneity could be improved. Also some special applications like imaging of the legs of a person typically demand at least four channels, as could be shown with multiple-channel research systems for MR imaging (MRI).
Such multiple-channel research systems are not yet available for the market, because they are expensive and complicated, since the production and the decoupling of the coils is very complicated. Due to residual coupling and patient dependent reflection, the known multiple-channel coils often need circulators to isolate power amplifiers for the different channels. Since circulators need a very defined magnetic field, it is difficult to place the power amplifiers near the coil or even at the coil. This design of the multiple-channel research systems is also different from the design of commercially available MRI-systems.